Exploring growth variability and extremes in Arctica islandica bivalve species in the context of large-scale climatic phenomena

Abstract

Hidden within the carbonate shells of Arctica islandica (A. islandica) bivalve species lies a wealth of information about past environmental conditions. Starting with the 1980s, this clam species surged not only in commercial popularity, but it quickly became one of the most important bio-records in the field of sclerochronology — which focuses on growth patterns in the hard tissues of accreting organisms — owing to individual lifespans exceeding centuries in the northern North Atlantic Ocean. Growth increments are typically annually deposited and can be precisely dated, thereby facilitating crossdating of multiple specimens to construct century-long master chronologies, and hence, long-term environmental records. However, compared to the isotopic composition of the growth bands, which acts as proxy for temperature, salinity and water productivity, growth increments are more challenging to decipher due to the interplay of multiple physical and biological factors, with the contribution of each factor varying in proportion depending on the region. With Arctica islandica already established in the community as a versatile climate archive, which novel insights can be envisioned? There is, for instance, an interest in exploring the spatial variability of environmental signals recorded by Arctica islandica master chronologies across diverse geographical regions. This attempt enables a more comprehensive understanding of the common growth signal and its relationship to large-scale climate phenomena. Spectral and correlation analyses have consistently revealed that A. islandica records decadal and multidecadal periodicities, often associated with natural oscillations that influence large-scale ocean circulation patterns such as the Atlantic Multidecadal Oscillation (AMO) and Pacific Decadal Oscillation (PDO), as well as atmospheric circulation patterns like the North Atlantic Oscillation (NAO). Given the ongoing state of climate change, characterized by increasingly frequent and potentially more intense extreme events compared to recent years, there is a burgeoning interest in assessing whether growth anomalies observed in marine organisms correspond to unusual climatic patterns. In my research, I define extreme growth as deviations in the 10th and 90th percentiles from the mean growth, and believe these can provide valuable insights into environmental stressors. The aim of this thesis and the results therein is to identify clues that can guide future research towards a deeper understanding of the complex large-scale climatic interactions shaping growth variability and extreme responses in A. islandica. To extract the shared growth signal in the northern North Atlantic, I have compiled previously published centennial-long master shell chronologies in several bivalve-based networks, and applied distinct principal component analyses. It was interesting to see a higher degree of congruity in the low growth years across the networks and the methodological approaches, underscoring the robustness of the findings concerning bivalve extreme growth. To identify anomalous large-scale patterns in the sea surface temperature and sea-level pressure, I have relied on composite maps of growth extremes along with reanalysis data. The composite analyses revealed that years marked by reduced growth aligned with positive sea-surface temperature anomalies in the extratropical to polar North Atlantic Ocean resembling multidecadal AMO mode, which paired with a coherent pattern in the North Pacific Ocean suggestive of a negative Pacific Decadal Oscillation. The bivalve networks further revealed that two chronologies from distinct areas — the northern Icelandic shelf and the German Bight in the southern North Sea — did not conform to the spatial patterns observed. Consequently, two case studies were conducted to investigate the unique characteristics of these locations. Using the chronology from the northern Icelandic Shelf, based on A. islandica bivalves from a water depth of 81 to 83 meters, my approach specifically addresses why correlations with sea surface temperature often lack the required statistical significance and are deemed inconclusive. In this example, I correlated the growth signal with a three-month running average of temperature and salinity data across various water depth layers, and found significant correlations with the subsurface waters starting at a depth of 56 m during summer and autumn, whereas correlations with the surface temperature layers were lagged by 2 years. Additionally, by computing spatial correlation maps, I highlight the potential of A. islandica to track thermally similar water bodies at large-scale. Next, I used an unpublished growth chronology of Arctica islandica from Helgoland in the southern North Sea alongside composite maps, to investigate which atmospheric patterns are associated with extreme growth events. The results indicate that growth in A. islandica is reduced during years characterized by a positive polarity of the NAO in winter, followed by intensified atmospheric blocking over the British Isles in spring. Spring blocking might also be linked to winter blocking in the North Pacific. The regional atmospheric pattern impacts the timing of the spring phytoplankton bloom, leading to a delay in the availability of essential nutrients necessary for growth. By bridging research gaps between the field of sclerochronology and climate dynamics, this thesis delves into different aspects of the Arctica islandica’s potential as a climate archive in the North Atlantic. Whether deciphering the common growth signal across bivalve networks (Paper I), or examining the spatial-depth relationship with subsurface temperature variations (Paper II), or uncovering links to atmospheric teleconnections and blocking frequency events (Paper III), each study adds exciting findings, holding relevance for both paleo- and future-oriented studies.